Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity.
Solar cells provide pollution-free, equal-access energy using a free natural resource. Due to environmental concerns and rising energy costs, solar cells, which may be composed of silicon substrates, are becoming more globally important. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Doping may improve efficiency of solar cells. FIG. 1 is a cross-sectional view of a selective emitter solar cell 210. It may increase efficiency (e.g. the percentage of power converted and collected when a solar cell is connected to an electrical circuit) of a solar cell 210 to dope the emitter 200 and provide additional dopant to the regions 201 under the contacts 202. More heavily doping the regions 201 improves conductivity and having less doping between the contacts 202 improves charge collection. The contacts 202 may only be spaced approximately 2-3 mm apart. The regions 201 may only be approximately 100-300 μm across. FIG. 2 is a cross-sectional view of an interdigitated back contact (IBC) solar cell 220. In the IBC solar cell, the junction is on the back of the solar cell 220. The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter 203 and the n+ back surface field 204 may be doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency.
In the past, solar cells have been doped using a dopant-containing glass or a paste that is heated to diffuse dopants into the solar cell. This does not allow precise doping of the various regions of the solar cell and, if voids, air bubbles, or contaminants are present, non-uniform doping may occur. Solar cells could benefit from ion implantation because ion implantation allows precise doping of the solar cell. Ion implantation of solar cells, however, may require a certain pattern of dopants or that only certain regions of the solar cell substrate are implanted with ions. Previously, implantation of only certain regions of a substrate has been accomplished using photoresist and ion implantation. Use of photoresist, however, would add an extra cost to solar cell production because extra process steps are involved. Other hard masks on the solar cell surface likewise are expensive and require extra steps.
The production of a solar cell requires many individual, sequential processing steps. Some of these steps may include:                Cutting/wire sawing of the silicon        Packaging/removal        Sorting        Cleaning/etching        Implant and anneal or diffusion        Application of an anti-reflective coating (ARC)        Application of metal contacts and firing        Inspection/cell testing        Cell sorting        
This list is not intended to be comprehensive and only serves to show the number of different steps which a solar cell must do through during production.
A primary goal of solar cell production is to produce the most efficient cells at the lowest production cost. Each of the above mentioned steps adds cost to the solar cell production process, as well as creating variability in the quality of the completed product.
To better understand the process, cell performance parameters, such as short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) as well as breakage, are typically monitored to maximize efficiency and minimize production cost.
Typically, substrates are tracked through the production process in “lots”. This may be suboptimal, since tracking large lots does not always give sufficient visibility to understand the specific causes for poor quality and defects. Furthermore, once a cell is separated from its lot, its traceability has been lost.
There are many methods for marking and tracking substrates that are currently available (laser etching, etc), primarily though various semiconductor chip manufacturing processes. The application of these marking methods to the solar cell process however is problematic. Many of the marking and tracking processes add to the production cost by requiring additional operations, increasing the overall production time.
In addition, most solar cell designs do not have a convenient surface that can be marked. The front of the cell, as shown in FIG. 1, is optimized for light collection and the back of the cell is typically used for the backside contact.
A low cost method to identify and track individual solar cells through the production process would be beneficial. Ideally, the method would maintain traceability of the cell through the entire product lifespan.